Thanks for you post and welcome to Mapsuite World! Would you please get the latest version 126.96.36.199 or 188.8.131.52 and try the following code:
RectangleShape shape = new RectangleShape(-180, 90, 180, -90);
Feature featuredfd = new Feature(shape.GetWellKnownBinary());
String geoJsonString = featuredfd.GetGeoJson();
if you have any more question , please feel free to let us know.